In the process to manufacture semiconductor devices, there is a trend that semiconductor wafers to be processed change from the current 6 or 8 inch wafers directly to 300 mm wafers. Thus, semiconductor manufacturing systems which can deal with 300 mm wafers have been developed. In 300 mm wafers, not only the diameter and the weight of the wafers have increased, but the width of lines in integrated circuits formed on the wafer have become ultraminute to not more than a sub-quarter-micron. Therefore, in semiconductor manufacturing plants, ultracleaning technology for clean rooms and automatic conveying technology for the wafers has become more important.
In the case of wafers up to 8 inches, the carrier for the wafers is conveyed usually with the wafers in upright attitude when conveying the wafers from one step to another, while the wafers are usually kept horizontal when conveying them into and out of processing units of the semiconductor production equipment. The operation of conveying the carriers into and out of the units is achieved by an operator or AGV (automatic guided vehicle). However, in the case of wafers of 300 mm, if the wafers are conveyed in an upright stance, the lower edges of the wafers may be damaged by the weight of the wafers, vibrations during conveying or the like. Therefore, the carrier is conveyed with the wafers kept horizontal. The operation of conveying into and out of the unit is difficult for the operator to achieve because of the weight of the wafers and the problem of generation of particles. Therefore an automatic operation is being promoted.
In the case of wafers up to 8 inches, after the load-lock chamber in which the carrier is placed is brought to a certain degree of vacuum, the wafers are conveyed one after another through a conveying chamber to a certain processing chamber. However, in the case of 300 mm wafers, the carrier capacity is large so that it takes much time to bring the load-lock chamber to a vacuum state. In addition, the vacuum causes, organic compound gases and the like are withdrawn from the plastic parts of the carrier, thereby polluting the inside of the unit with impurity gases. Therefore, in processing systems which deal with 300 mm wafers, a cleaning chamber is provided between a carrier-housing chamber and a load-lock chamber so that the wafers are first conveyed into the load-lock chamber from the carrier positioned in the carrier-housing chamber by a conveying unit placed in the load-lock chamber. The wafers are then conveyed through a conveying chamber into each processing chamber one by one. Thus carriers adapted for 300 mm wafers are required. At the present time, carriers can be roughly classified into an open-type carrier and a closed-type pod (for example, an unified pod) which is closed with a lid after putting the carrier therein.
For example, as shown in FIG. 4, the above processing system comprises a tray 1 for receiving a pod P which houses 13 or 25 wafers, a carrier-housing chamber 2 for housing the retracted tray 1 on which the pod P is put, and a load-lock chamber 3 having a wafer conveying unit (not shown) which conveys wafers in a bunch into and out of the pod P in the carrier-housing chamber 2. Furthermore, a cleaning chamber 5 is provided between the carrier-housing chamber 2 and the load-lock chamber 3 to clean the atmosphere of the wafer passage line. To the load-lock chamber 3 is connected a conveying chamber 4 via a gate-valve so that the wafers in the load-lock chamber 3 are conveyed into the processing chamber for wafers (not shown) through the conveying chamber 4 one by one. This processing system is provided in a unit area R2 which is divided from a common area R1 by a front panel 6 in a clean room R.
At the ceiling of the clean room R, a high-performance filter F such as a ULPA filter or a HEPA filter is placed so that air cleaned by the high-performance filter F flows downward in the clean room R. This processing system introduces air in the unit area R2 into the cleaning chamber 5 by a suction fan (not shown) provided in the cleaning chamber 5; cleans the air with a high-performance filter provided on a downstream side of the suction fan, similar to the above high-performance filter; causes the air to flow downward in the cleaning chamber 5 as shown by arrows in FIG. 4; and discharges the air from the bottom to the outside. In the cleaning chamber 5, an opener 7 is arranged as shown in FIG. 4. This processing system opens the lid of the pod P by the opener 7, conveys wafers W in the pod P in a lump through the cleaning chamber 5 into the load-lock chamber 3 by the wafer conveying unit in the load-lock chamber 3, keeps all the wafers W horizontal in the load-lock chamber 3, and conveys the wafers held by the conveying unit through the conveying chamber 4 into the processing chamber. At each transfer port of the wafers in the load-lock chamber 3 and in the conveying chamber 4, a gate-valve (not shown) is arranged.
In the case of the above described conventional processing system, there are some problems owing to introducing the air in the unit area R2 into the cleaning chamber 5. In the unit area R2, unlike the common area R1, various units are ordinarily provided. Consequently, particles may be produced from these units or organic gases such as an organic solvents consisting by of hydrocarbons may be produced as impurity gases from the painted portions of these units. Because of this, the air in the unit area R2 becomes polluted by these impurities, and the air in the unit area R2 is inferior in cleanliness to that in the common area R1. If the air in the unit area R2 is introduced into the cleaning chamber 5 as it is, even though the particles can be removed by a high-performance filter in the cleaning chamber 5, the impurity gases can not be removed, so that the impurity gases may adhere to the surfaces of the wafers W, thereby disturbing the processing of the wafers W and reducing the yield in the process. The more ultraminute the processing of wafers W is, the greater influence of particles and the impurity gases. In addition, if air is introduced, oxygen in the air produces oxidation films on the surfaces of the wafers W and moisture in the air flows through the cleaning chamber 5 into the load-lock chamber 3 to adhere to the walls of the load-lock chamber 3. This brings about an adverse influence in that the length of time needed to bring the load-lock chamber 3 to a vacuum state becomes long.